In the paper industry, one currently carries out the recovery of old papers in the following manner:
The old papers are first dispersed in water and reduced into pulp in apparatus called pulpers operating a rough cleaning by elimination of the heavy parts of density higher than that of the pulp (a density of about 1.027).
The pulpers operate in general at a low concentration, less than 5%.
At the outlet of the pulper, the pulp is sent in at least one screen or strainer, operating a screening with a sieve. The object of this operation is to clear the pulp from the wood, plastics and metal contaminants it may contain. Generally, a series of two apparatus has to be used, one of them comprising at least one fine slit sieve which eliminates all the elements having a particle size exceeding the width of the slits, while letting through the fibres and the flat and very thin elements; the other apparatus comprises at least one hollowed sieve eliminating the flat elements.
It is usual to operate with an apparatus comprising a sieve with holes having a diameter of the order of 4 to 5 mm, followed by an apparatus comprising a sieve having slits of a width of the order of 0.3 to 0.4 mm.
At the outlet of these apparatus, called screens or strainers, and which will be for example of the type discussed in French Pat. No. 77.35151 filed on the Nov. 23, 1977, a cleaned pulp containing water and fibres with the exclusion of contaminants is obtained on one side, and refusals or rejections with a high content of contaminants is obtained at the other side.
By way of example and with an apparatus such as that discussed in French Pat. No. 77.35151 into which is sent a pulp having a concentration of the order of 3 to 5% of dry materials and containing about 5% of pollutants, there is obtained at the outlet:
on the one hand a pulp containing 0.05% of pollutants with a concentration which is always of the order of 3 to 5% of dry materials: PA1 on the other hand a rejection containing about 30% to 50% of pollutants, the remainder being pulp with a concentration of the order 3 to 5% of dry materials. PA1 there is foreseen an additional water inlet in the inner chamber, on the side of the pulp arrival, PA1 the helical outer chamber is placed such that it is wrapped around the fixed sieve from the pulp inlet to the refusals outlet, with a rotation direction consistent with that of the liquid in said outer chamber, PA1 this rotation direction is the direction contrary to that of the rotor rotation direction.
One sees that the rejections contain appreciable quantites of pulp and that they are highly diluted. Thus, said rejections cannot be discarded and they are very cumbersome.
The presently most prevalent process for treating them consists in sending them into auxiliary apparatus usually called "sorters" which are generally vibrating sieves in which the rejections are washed in a high diluted state and in an open field thereby allowing obtaining on the one hand washed rejections and on the other hand a very diluted pulp intented for being recycled.
Such apparatus are affected with serious disadvantages, first of all because, by operating in an open field, they require a pumping system and a supply of clean water; they are therefore consumers of energy and water and produce very diluted products which have to be thereafter reconcentrated; on the other hand, as all vibrating apparatus, they require large masses of support concrete for absorbing the vibrations, and their maintenance costs are high; finally, if the cleaning has been carried out by means of fine slit sieves, the washing of the rejections is practically inoperative since a vibrating sieve is necessarily a hollowed sieve, the slit sieves not offering a sufficient mechanical strength for equipping vibrating apparatus. Due to this fact, a large part of the rejections eliminated by the strainer get through the vibrating sieve and are recycled.